Presentation on theme: "Heavy Concrete Construction Formwork"— Presentation transcript:
1Heavy Concrete Construction Formwork Unit 68Heavy Concrete Construction FormworkPlywood Forms • Foundation and Basement Forms • Patented Ties and Waler Systems • Pilaster, Column, and Beam Forms • Floor and Roof Forms • Formwork Construction • Stairway Forms • Prefabricated Panel Forming Systems
2An advantage of plywood in form construction is that it can be bent when a curved surface is required.Most wood forms today are constructed of plywood back-ed by various combinations of studs, walers, strong-backs, and some type of bracing. Plywood and com-posite panels provide a large, smooth surface that can withstand rough treatment without splitting. Another advantage of plywood is its ability to bend when curved surfaces are required in the form design. See Figure 68‑1. Textured plywood can also be used for a textured finish on the concrete.
3A release agent is sprayed on panels to be used for concrete forms to facilitate form removal. Follow manufacturer instructions for the safe use and application of form-release agents.After the formwork panels have been stripped and clean-ed of any sediment that has adhered to the panels, a form-release agent should be applied. Spraying is the most common means of applying form-release agents. See Figure However, manufacturers may recom-mend that their form-release agents be applied using a mop or a cloth.
4Outside form walls are erected and rebar is placed prior to constructing inside form walls. In constructing basement wall forms, the first step is to place all the outside panel sections. See Figure If possible, holes for snap ties should be drilled in the panels before they are erected.
5Wood braces are equipped with turnbuckles at their lower ends to allow wall panels to be aligned. Turnbuckles are fastened to the ground with steel stakes. A brace plate is used to attach a brace to a waler or stiffback.After all wall panels have been positioned and the walers and strongbacks (if required) have been attached, the wall is aligned and braced. Braces are usually placed 6′ to 8′ apart. Braces may be nailed to the wall and to wood stakes driven into the ground, or patented braces may be used. One type of patented wood brace features a turnbuckle that is welded to an anchor bracket, which is fastened to the ground with steel stakes. See Figure 68‑4.
6All‑steel wall braces may be used to support wall forms All‑steel wall braces may be used to support wall forms. Braces should be properly positioned to provide adequate support.A brace plate is used to fasten the brace to a waler or metal strongback on the wall. Another type of brace is made completely of steel. See Figure 68‑5.
7Metal window frames are secured to outside form walls as the walls are erected. The frames remain in place after the form walls are removed.After the outside wall form panels for the basement walls are set, provisions are made for door and window openings. Steel or aluminum window and door frames are commonly attached to the wall form panels. Brackets extending from the frames will be embedded in the concrete to secure the frame in position when the forms are stripped. See Figure 68-6.
8Door and window bucks use similar construction techniques Door and window bucks use similar construction techniques. Door bucks do not require a bottom piece with an inspection pocket.A traditional method for providing for door and window openings is to set wood window and door bucks as shown in Figure 68‑7. A rectangular opening at the bottom allows observation of the flow and consolidation of concrete beneath the buck. When concrete reaches the bottom of the buck, the piece that has been cut out is replaced and cleated down. The entire buck is removed when the forms are stripped from the hardened concrete walls. The wedge‑shaped key strip remains in the con-crete and serves as a nailing strip for the finish wood window frame.
9Rebar is placed after the outside form walls are set and before the inside form walls are erected. Reinforcing steel bars (rebar) are placed by reinforcing-steel workers after the outside wall forms are set but before the inside wall forms are erected. See Figure 68‑8. With some forming systems, rebar is placed first and the forms are built around the rebar.
10Bulkheads may be required in long walls Bulkheads may be required in long walls. Bulkheads are made of 1 × 8s or 2 × 4s which are notched around the rebar.It may be necessary to place concrete for long walls in sections, creating vertical construction joints. When constructing long walls, carpenters must place a bulkhead inside or at the end of the form. See Figure 68‑9. Depending on the wall thickness, bulkheads are made of 1 × 8s or 2 × 4s, which are notched around the rebar. In some situations, the bulkhead members are placed along the sides of the rebar and the spaces between the members are filled.
11Form walls are doubled up after the rebar, door and window frames, and utilities have been properly located.After the outside form walls are set in position, rebar is placed, and required door and window bucks and utilities are installed, carpenters double up the walls by setting the inside wall forms in position. The bottoms of the inside wall panels are placed in the proper position and the panel is slowly tilted into the vertical position. Form ties extending from the outside form wall are inserted through predrilled holes in the inside form wall panels. Inside form wall walers are then attached. Clamps, edges, or other devices are placed at both ends of the ties, spacing and fastening together the opposite form walls. See Figure 68‑10. Some types of ties are pushed through the panel holes after the walls have been doubled up.
12Snap ties are used to secure and properly space opposing form walls. Small plastic cones or metal washers on snap ties act as spacers between the walls. A break-back consisting of a grooved section next to the tapered edge of the cone allows the tie to be broken off (snapped) after the con-crete has hardened and the forms have been removed. A variation of a snap tie design is a system that features a nut button that enables the tie to be snapped by turning the button with a socket wrench while the forms and wedges are still in place. See Figure 68‑11.
13Internal disconnecting ties are commonly used on heavy walls. Medium to heavy ties are internal disconnecting ties and have safe working load values exceeding 3750 lb. Inter-nal disconnecting ties, such as waler rods, coil ties, and taper ties are used for heavier and thicker walls where greater pressure is exerted during concrete placement. See Figure
14Heavy construction panel forming methods commonly use 2 × 4 studs, which are reinforced by walers. Note the snap tie wedges along the walers.Traditional heavy construction panel forming methods use vertical 2 × 4 studs spaced 12″ to 16″ OC. See Figure 68‑13. The studs are reinforced by double or single walers and/or strongbacks, depending on design considerations. Snap ties, waler rods, and coil ties are commonly used with this system.
15Studs are not required for some single‑waler systems supported by strongbacks. One forming method featuring a single waler system eliminates the need for studs beneath the walers. After the wall form panels are set in place, snap brackets and walers are attached. Wedges secure the walers in place. Strongbacks and braces are then placed at intervals. See Figure 68‑14.
16Pilasters provide additional strength to a wall and support the ends of beams. After the wall forms are positioned, walers and studs will be placed.If a wall design includes pilasters, pilaster forms are erected as the wall forms are constructed. See Figure 68‑15. Pilasters are projections from the face of a wall which add strength to the wall and may also support the ends of beams.
17When assembling and placing wood column forms, tight joints and strong tie supports around the form are necessary. A cleanout hole is provided at the bottom of one of the sides.When wood forms are used for rectangular columns, the sides are constructed of plywood and backed with 2 × 4 stiffeners. Stiffeners may be omitted for smaller columns that require a lighter form assembly. A cleanout hole is provided at the bottom of one of the sides. After the sides are assembled, they are placed in a template fastened to the column footing. See Figure 68‑16.
18Scissor clamps are tightened and held in place by cam devices or wedges driven into slots in the clamp.Hinged metal scissor clamps are often used to tie to-gether the column form sides. The scissor clamps are tightened and held in place by cam devices or wedges driven into slots located in the clamp. See Figure 68‑17. After the concrete has hardened and the forms have been removed, the corners may be temporarily protected with wood strips.
19Tubular fiber forms for round columns have been set in place and braced. Rebar will be set inside the tubes, and then concrete will be placed.Tubular fiber forms are frequently used to construct round concrete columns. The tubular forms are posi-tioned and secured in position with wood or metal braces. When stripped, the forms are cut with a knife or saw and carefully pried from the concrete. See Figure 68‑18. Tubular fiber forms can be easily cut to length on a job site using a handsaw.
20Column spring forms are stripped by removing the flange bolts and pulling apart the flanges. One‑piece molded fiberglass forms are also used for the construction of round columns. The fiberglass forms are pulled apart and placed in position around previously installed rebar. The edges of the form are then secured with bolts at closure flanges. The form is then secured in position with braces tied to a steel bracing collar. Forms are stripped after the concrete has set by removing the flange bolts and separating the flanges. See Figure 68‑19. Other form systems made entirely of steel are also used for the construction of rectangular and round columns.
21A structural frame for a concrete building may include columns, beams, and girders. Forms for beams and girders rest on top of and are tied to the column forms. Although the terms “beam” and “girder” are often used interchangeably, they have distinct meanings. A beam is a horizontal member that supports a bending load over an opening, as from column to column. A girder is a heavy beam that supports other beams and girders. See Figure 68‑20.
22Columns, girders, and beams may be formed as a single (monolithic) unit. Note that the beam and girder join over the column.Beam and girder forms consist of a bottom piece (soffit) and sides. See Figure 68‑21. The entire unit is sup-ported by shores placed at intervals. Patented metal shores are commonly used to support the forms, although wood T-shores can also be used. Beams and girders are heavily reinforced with rebar that tie into the rebar of the column below and the floor above.
23Concrete is placed at the same time for the walls, columns, beams, and girders of a monolithic unit. An example of beam‑and‑column concrete construction is shown in Figure 68‑22.
24This concrete floor system features a slab resting on concrete joists that tie into girders. Beam‑and‑slab systems are suitable for floors that will bear heavy loads. The floor slab rests on top of closely spaced beams that tie into girders supported by columns. A lighter design features concrete joists tied into girders. See Figure 68‑23.
25A flat‑slab floor may be supported by drop panels over the columns. Beams and girders are not used with a flat‑slab system. The slab receives its main support from the columns and the thickened sections over the columns called drop panels. See Figure 68‑24.
26A flat‑slab floor may be supported by drop panels and capitals over the columns. A variation of the drop panel system uses column capitals, sometimes called drop heads, over the column. See Figure 68‑25.
27Long pans used for one‑way joist systems rest on 2″ thick soffits supported by shores and stringers. Long pans are used to form one‑way joist systems. The pans are nailed to the tops or sides of 2″ thick sof- fits, which are supported by shores and stringers. See Figure 68‑26. Long pans commonly frame into girder forms.
28Fiberglass dome forms are in place for a two-way joist system Fiberglass dome forms are in place for a two-way joist system. Here, the worker is applying compressed air between the concrete slab and dome forms to loosen them.Dome pans are used to form two‑way joist systems and are supported in the same manner as long pans. Since two‑way joists do not include beams or girders, a solid area equal to the slab thickness and joist is formed around the supporting columns. See Figure 68‑27.
29Tubular shores and metal beams are commonly used with heavy construction formwork. Slab decks are formed over metal, engineered wood, or solid wood shores, depending on the deck design and the load imposed on the deck. Metal (steel and alumi-num) tubular shores are commonly used with heavy construction formwork. See Figure
30Metal scaffold shoring is used to support high form soffits. Metal shores can be reused many times, and offer the convenience of screw jacks for making height adjust-ments. To support higher soffits, metal scaffold shores may be employed for greater stability. See Figure
31Engineered wood products, such as laminated veneer lumber (LVL) beams and wood I-joists, provide proper support for form soffits.Engineered wood products, such as veneered beams and I-joists, are also commonly used to support form soffits. See Figure Aluminum beams are also frequently used.
32Slab decks may be supported by wood shores and stringers Slab decks may be supported by wood shores and stringers. Wedges placed under the shores or screw jacks attached to the shores provide for vertical adjustment.A traditional method of shoring slab deck soffits involves the use of wood shoring. The shores are cut short to allow for wedges over a wood sill. See Figure The wedges are used to drive the shores up tightly and to line up the floor above. Metal shore jacks can also be used for this purpose. Horizontal braces are nailed or screwed to the posts at mid-height to tie the posts together. Plywood cleats secure the stringers (beams) to the shores. Stringers are then set on top of the shores and joists are laid across the stringers. The plywood deck is then nailed to the joists.
33Rebar is placed over the deck of the floor form Rebar is placed over the deck of the floor form. Note the opening for the beam extending the length of the deck.When the deck formwork has been completed, rebar is placed over the deck and properly positioned. The floor rebar are tied to rebar in the walls, beams, and columns. See Figure 68‑32.
34When constructing first‑floor heavy construction forms over foundation footings, wall forms are first placed, followed by column, beam, and deck formwork. Note reinforcing steel is not shown.The general sequence for constructing hand-set forms for the first floor of a building is shown in Figure 68‑33. The procedure for setting forms for subsequent floors is similar.
35Rebar for columns usually extends past the floor and beams that are supported by the columns. Rebar extending from this column will be tied to additional steel placed inside the form for the column of the floor above.When rebar is placed in walls and columns, it will extend above the next floor level. The rebar may project several feet above floor level or possibly to the top of the second-floor walls and columns. See Figure 68‑34.
36An outside wall panel is fastened to a previously placed floor using anchor bolts which are embedded in the floor slab.When forms are constructed for the outside walls of the second story, the bottoms of the outside form wall panels must be secured to the top of the first-story wall. Anchor bolts are embedded in the concrete of the previously placed wall. See Figure 68‑35. The anchor bolts extend through walers at the bottom of the panels. Large wash-ers and nuts are tightened over the walers.
37Rebar extending from the floor level above and below is tied to rebar in the stairway. Construction of a concrete stairway form, however, can be more complicated than construction of a wood stair-way. On heavy construction projects, the architect may indicate the unit run and unit rise, as well as a permissible tolerance. The unit run and unit rise are then field verified on the job and can be adjusted as necessary. Figure 68‑36 shows a procedure for laying out and constructing an open stairway form.
38A closed concrete stairway is built between two walls. Figure 68‑37 shows a procedure for laying out closed concrete stairway forms.
39Prefabricated gang forms commonly include walers, strongbacks, lifting brackets, and scaffold brackets. Note the toprails, midrails, and toeboards in place to protect workers against falls and prevent materials from falling from the platforms.On structures where concrete must be placed in two or more stages, gang forms are often used. Gang forms are large panels constructed by fastening together a series of smaller panel forms. Walers, strongbacks, lifting brack-ets, and scaffold brackets are then fastened to the panels. The entire unit is lifted and placed in position by crane. See Figure 68‑38. When the concrete has set sufficiently, the forms are then removed and moved to another location.
40The support for a flying floor form unit is provided by aluminum trusses placed on either side and aluminum beams placed across the trusses. A plywood deck is fastened on top of the beams. Adjustable jacks are used to raise the unit into position.Flying forms are prefabricated forming systems that usually consist of a wood deck and a metal support system. See Figure 68‑39.
41Flying forms used for floors or other structural members are set into place by crane. Flying forms may be used to form floor slabs and other structural members such as beams. The forms are placed in position with a crane. See Figure After placing concrete and allowing it to harden sufficiently, the entire flying form is removed and lifted to the next floor level. The use of flying forms greatly increases pro-duction on multistory buildings.
42Slip forms are commonly used in the construction of tall buildings Slip forms are commonly used in the construction of tall buildings. Concrete is transported to the upper level of the building and is placed in forms using a pumping apparatus.Slip forms were originally developed for the construction of curved concrete structures such as silos and towers. Slip forming methods have expanded to the construction of rectangular buildings, caissons, building cores, un-derground shafts, shear-wall buildings, communica- tion towers, and a variety of other structures. See Figure 68‑41. Slip forming can save significant labor, time, and material cost on construction projects.
43The basic design of a standard slip form includes jackrods, hydraulic jacks, cross beams, and yoke legs. Additional features, such as scaffold, are custom made for the structure being erected.Most slip forms consist of 4′ high inner and outer walls of 3/8″ to 3/4″ plywood panels supported by a 2 × 4 studs and 2 × 6 walers. Inner and outer form walls are slightly tapered outward at the bottom (1/8″ per foot) to reduce the amount of drag on the concrete as the form is raised. The walls are secured together with steel cross beams and yoke legs. Cross beams tie together the tops of opposing yokes and provide a mounting surface for the hydraulic jacks. The yoke legs are made of steel and are spaced approximately 6′ apart along the length of the slip form. The yoke legs are adjusted to the wall width and are fastened to the cross beams at the top end and to the walers along the bottom end. Hydraulic jacks are mounted on the cross beams. Slip forms are raised by electrically or pneumatically powered hydraulic jacks that climb jackrods extending into the form. Perfect coor-dination of the hydraulic jacks is essential for accurate alignment of the forms; all hydraulic jacks must be lifted at the same time and at the same rate. Jackrods are threaded at each end. Additional lengths of jackrod are fastened to the upper threaded end when required. See Figure 68‑42.